(a) Field of the Invention
The present invention relates to an air-fuel ratio control mechanism for a fuel injection apparatus of the type which uses a servomechanism utilizing a fluid to control the fluid-communication ratio of a fuel measuring gate disposed in a fuel supply passage, wherein a sensor signal which detects the operating condition of the engine or a simulation signal which senses the operating condition of the engine is used to provide an air-fuel ratio optimum for the operating condition of the engine, namely, an ideal air-fuel ratio.
(b) Description of the Prior Art
As for techniques prior to the present invention, there are those which the present applicant applied for patent in Japan, and which are illustrated in FIGS. 1 and 2, respectively.
A fuel injection apparatus A shown in FIG. 1 comprises a servomechanism B using a fluid for maintaining at a predetermined value a pressure difference P.sub.1 -P.sub.2 across a throttle valve 2 disposed in a suction pipe 1, and a fuel measuring and distributing mechanism C for measuring the amount of air being sucked into the internal combustion engine on the basis of the degree of opening of said throttle valve 2 and establishing unique correspondence between the degree of opening of the throttle valve 2 and the fluid-communication ratio, namely the opening time and/or area of opening, of a fuel measuring gate 3, 4 disposed in a fuel supply passage to maintain at a predetermined value a pressure difference PF-P.sub.i at the measuring gate 3, 4 for measuring the amount of fuel relative to the amount of suction air, said fuel injection apparatus A being designed so that the set value for said pressure difference P.sub.1 -P.sub.2 across the throttle valve 2 (hereinafter referred to as the basic set value for the servomechanism is compensated by the output from a sensor 5 which detects the residual oxygen concentration in the exhaust gas. In this case, as an interface for control, use is made of a heater 8 enclosed in a bellows 7 interlocked to the pressure difference setting diaphragm 6 of the servomechanism B or use is made of a heater (not shown) associated with a bimetal disposed adjacent the pressure difference setting diaphragm 6.
An apparatus shown in FIG. 2 is designed so that the degree of opening of the valve 10 of a pressure regulator 9 which maintains at a predetermined value the difference between the supply pressure of fuel being supplied to a fuel measuring and distributing mechanism C and pressure in a suction pipe is controlled by a solenoid value 11 adapted to be actuated by the output from an exhaust gas sensor (O.sub.2 sensor) 5, whereby the fuel supply pressure PF is varied to control the air-fuel ratio.
However, in the former apparatus, it takes much time for the heater 8 to be heated to vary the pressure in the bellows 7 or deform the bimetal, so that the speed of response to a variation in the conditions is low and particularly the ability to follow up transient conditions is poor, while in the latter apparatus, although the speed of response is high, the engine torque is liable to vary if the regulatable range is widened, so that there is a danger of the car driver to lose his sense of driving.